Neuromuscular reflex plays a central role in the maintenance of muscle tone and hypertonia forms a basis of muscle contracture. As a sensory organ for muscle length in the peripheral neuromuscular reflex loop, muscle spindle produces positive feedback (la and II afferent) to simulate alpha-motor neuron activity. The sensitivity of a spindle is filtered by the tension of intrafusal muscle fibers under gamma-efferent regulation. Much attention has been paid to the spindle function in muscle function and spasticity and the contractility of intrafusal fibers is an essential link in the reflex loop. The intrafusal fibers contain unique myosin isoforms as compared with the extrafusal fibers, but little is known for their Ca 2+ regulation and contractile features. The regulation of intrafusal myofilament protein isoform expression during muscle development, adaptation and diseases is largely unknown. Based on our previous studies, we plan to investigate the role of myofilament protein isoforms in neuromuscular reflex. Our research plan is focused on testing a hypothesis in which the changes in fiber type-specific myofilament protein isoforms, especially the actin filament-associated regulatory protein troponin T (TnT), in intrafusal fibers may play a role in the pathophysiology of muscle contracture. It has been found that spastic muscles have increased type I (slow) fibers. Cerebral palsy, joint immobilization and tenotomy, three very different original conditions which cause muscle contracture, have a common consequence that is a fixed shortening of the resting muscle length. We have found an increased expression of slow myosin in a tenotomy model and the expression of myosin and thin filament regulatory protein isoforms is coordinated in the muscle. As an acidic TnT isoform, an up-regulation of slow TnT would increase the sensitivity of myofilaments to Ca2+ activation. The increase in intrafusal fiber Ca2+ responsiveness will increase spindle tension and sensitivity, which in turn increases the positive feedback to stimulate alpha-motor neuron to activate the extrafusal fibers and result in hypertonia. To test this hypothesis will help to understand the pathophysiology of muscle contracture. Three specific aims will be pursued in this pilot study: I. To examine the thin filament regulatory protein isoforms expressed in intrafusal fibers in adult and developing muscles. II. To investigate whether fixed shortening of muscle length originated from different conditions induces similar changes in the expression of intrafusal myofilament protein isoforms. III. To test whether elevated slow TnT expression in transgenic mouse muscles will produce increased Ca2+ sensitivity of intrafusal fibers and increased alpha-motor neuron activity. To explore this largely unknown area of neuromuscular reflex, this research initiative will lay groundwork for understanding the molecular mechanism of muscle contracture and improving treatment of this disabling condition.